1. Field of Invention
The present invention relates generally to RF detectors and, more particularly, to RF detectors capable of providing an indication of both the average power and the instantaneous or peak power levels of an RF signal.
2. Discussion of Related Art
There are many applications in which it is desirable to measure the peak and/or average power level of a radio frequency (RF) signal. For example, power measurement and control of the RF signals in both the transmitting and receiving chains of modern wireless communications systems, such as cellular telephone networks, may be essential. To efficiently use the available bandwidth, the transmitted signals in these systems may be modulated using complex modulation standards such as CDMA, WCDMA or WiMax. These complex modulated signals have a time varying crest factor, which is defined as the peak to average power ratio of the signal, resulting in intolerable errors if conventional power detectors using diode detection or successive amplification are used to measure signal power.
Conventional techniques to characterize modulated signals depend on the parallel processing of the input signal to compute the average power and the instantaneous or peak power. For example, referring to FIG. 1A, a single RF input signal at terminal 102 is processed by an envelope detector or peak detector 104 to generate the instantaneous power/peak power output, a, and is also processed by an average power detector 106 to generate the average power output, b. FIG. 1B illustrates a variation of this method in which the RF input signal is initially processed using a power envelope detector 112 and is then processed using an averaging circuit 114 to generate the average power output, b, and using the buffer/peak detector 104 to generate the instantaneous/peak power output, a.
In some cases, it is desirable to measure the crest factor of the RF signal and/or to obtain information about the signal wave shape. Calculation of the crest factor requires both average power information and peak power information. Referring again to FIG. 1A, a conventional technique uses a divider 108 to calculate the crest factor after parallel processing of the RF input signal to determine the peak power, a, and the average power, b, as discussed, for example, in U.S. Pat. No. 5,220,276. The divider 108 calculates the ratio of the peak detector 104 output (a) to the average power output, b, resulting in an output signal (b/a) on terminal 110 that is a representation of the input signal crest factor. When an envelope detector is used instead of a peak detector, the divider provides an instantaneous power output signal that is normalized to average power; i.e., the ratio of instantaneous power to average power.
A disadvantage of RF detectors using the parallel processing technique is that an RF coupler (not shown) is required at the input 102 to drive both the average power detection channel and the envelope power or peak power detection channel. In addition, because different detectors (104 and 106) are used in the two different detection channels, there may exist part-to-part, process, and temperature variations between the two channels, which can degrade the accuracy of the measurements, particularly of the crest factor measurement. Matching circuitry may be required to compensate for such differences between the two channels, adding complexity and cost to the system. Another disadvantage is that the divider 108 may be required to handle nonlinear calculations depending on the output characteristics of the average power detection channel and the envelope power or peak power detection channel. In addition, because any inaccuracy in the divider would compromise the crest factor measurement, an accurate, possibly complex and expensive, divider may be required.
There are a variety of commercially available average power detectors that may be used in the systems illustrated in FIGS. 1A and 1B. One example of an average power detector is an RMS-DC converter. RMS-DC converters are used to convert the RMS (square-root of the mean (average) of the square) value of an input AC (time-varying) signal into a DC (or quasi-DC) current or voltage. RMS-DC converters are capable of measuring the RMS power of an RF signal independent of the signal wave shape or crest factor. Wide-dynamic range average power detectors using feedback control loop techniques are commercially available.
For example, referring to FIG. 2, there is illustrated a block diagram of an RMS-DC converter 200 that incorporates a squaring RF detector 206 with a variable scale factor and a feedback control loop. The RMS-DC converter receives an RF input, Vin, at terminal 202 and provides a signal at terminal 204 which is representative of the average power of the input signal. The squaring RF detector 206 is responsive to a scaling factor control signal, Vscale, received at a control port 208 and provides an output voltage, Vout, at an output port 209, the output voltage being a representation of the square of the RF input signal scaled by a monotonic function of the control signal. Thus, the output of the squaring RF detector 206 is given by:Vout=|Vin|2×ƒ(Vscale)  (1)
The second element of the average power detection feedback loop is an integrator 210 having an input port 212 coupled to the output port 209 of the squaring RF detector 206, a reference port 213 that receives a reference signal 214, and an output port 216 coupled to the control port 208 of the squaring RF detector 206. The output port 216 of the integrator 210 is also coupled to the terminal 204 of the RMS-DC converter 200. The integrator 210 is configured to integrate the difference between the output, Vout, of the squaring RF detector 206 and the reference signal 214 to adjust the scaling factor of the squaring RF detector until the average output signal of the squaring RF detector is equal to the reference signal, thus resulting in a feedback control loop. This feedback loop forces the squaring RF detector 206 to operate at a controlled output operating point. For example, a drop in the RF input signal power received at terminal 202 results in negative integration in the integrator 210, forcing the squaring RF detector 206 to provide amplification to the input signal to keep the average squaring RF detector output, Vout, at a constant point. Because of this interaction in the feedback control loop, the scaling factor control signal, Vscale, of the squaring RF detector 206 will vary as a function of the average of the RF input signal, Vin, providing a representation of this RF input signal average power. Some examples of such, or similar, RMS-DC converters are disclosed in U.S. Pat. Nos. 6,348,829, 6,429,720 and 6,437,630.
Single-detector average power or peak power detecting schemes, such as those illustrated in FIGS. 1A and 1B, suffer from a reduced dynamic range, for example, on the order of about 35 dB in high-frequency integrated circuit detector designs. Average power detector using a feedback control loop technique, such as that illustrated in FIG. 2 and discussed above can achieve much larger dynamic range depending on the scaling function implementation in the feedback control loop. For example, average power detectors with more than 75 dB dynamic range are commercially available. However, in a system such as that illustrated in FIG. 1A, where both peak/envelope and average power are measured, the normalized instantaneous power output or peak power output would be limited by the lesser performance (dynamic range) of the envelope power or peak power detecting scheme. In addition, single-detector envelope power or peak power detecting scheme are generally also highly dependent on the input RF frequency, which may not be desirable in many applications.